The optical response of InAlAs/InGaAs HEMT's under illumination with modulated light from a 1.3-μm semiconductor laser diode onto the backside of the substrate is measured by using an optical-signal analyzer. It is clear that the response is composed of two signals. One signal is dominant at a low frequency and is due to the photovoltaic effect that causes excess holes photogenerated in the InGaAs channel to accumulate in the source region. This accumulation thus causes a decrease in the threshold voltage of the HEMT's. To explain this mechanism, a theory is given which connects the change in threshold voltage with that in the Fermi energy of the two-dimensional electron gas (2-DEG). The other signal is dominant at a high-frequency and is due to the photoconductive effect in the InGaAs channel beneath the gate. In this case, a large optical gain is produced since electrons at the source region are replenished in the gate channel. This leads to the first clear observation of a photoconductive signal. The bandwidth due to the photovoltaic effect is as low as 45 MHz and is dominated by the lifetime of the excess holes. The bandwidth due to the photoconductive effect is as high as 37 GHz and is dominated by the gain-bandwidth product of transistors rather than the intrinsic transit-time of electrons.
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